Twin sheet thermoforming processes typically involve providing two separate preformed sheets of thermoplastic material (each usually on a separate roll), heating each of the preformed thermoplastic sheets to a thermoformable temperature, and contacting each of the heated sheets with a separate shaped mold surface, thereby forming shaped sheets on the mold surfaces. The molds are then brought together so as to contact and fuse portions of the shaped sheets together, resulting in formation of a molded article.
Such prior art methods of twin sheet thermoforming typically and undesirably involve multiple steps, such as separately forming the thermoplastic sheets, collecting the preformed thermoplastic sheets on separate rolls, transporting (e.g., shipping) the rolls of preformed thermoplastic sheet to a molder (or fabricator), and re-heating each of the preformed thermoplastic sheets prior to the thermoforming and fusing operations. Such a multiplicity of steps involves the additional consumption of energy, which increases both the economic cost and carbon footprint associated with the process and the molded articles formed thereby.
In addition, the re-heating step may result is uneven heating of one or both of the preformed thermoplastic sheets, which may then result in defective formation of one or both of the shaped sheets, and/or defective fusing of the shaped sheets together. For example, a preformed sheet that has been unevenly re-heated may not properly conform to the shape of the mold when drawn into contact therewith, resulting in structural and/or aesthetic defects in the shaped sheet, (e.g., cracks, tears and/or thickness defects, such as thinning in over-heated areas and thickening in under-heated areas, irregular coloration and/or discoloration). Substandard fusion may alternatively or additionally occur between the contact points of unevenly re-heated preformed sheets, resulting in molded articles having fusion area defects that may undergo catastrophic failure (e.g., cracking and/or separation) when exposed to static and/or dynamic loading.
U.S. Pat. No. 7,045,086 B2 discloses a twin sheet thermoforming process and carousel apparatus that involves simultaneously heating two preformed thermoplastic sheets in a single heating station, rather than sequential heating of the preformed sheets.
Single sheet thermoforming processes that involve the continuous extrusion of a thermoplastic sheet, that is thermoformed using residual heat from the extruded thermoplastic sheet are known. See, for example, U.S. Pat. Nos. 6,814,905 B1, 6,086,800 and 4,061,706.
It would be desirable to develop new twin sheet thermoforming processes, and apparatuses used therewith, that minimize or eliminate the multiple steps typically encountered with prior art methods. In addition, it would be further desirable that such newly developed methods and apparatuses also provide molded articles having physical properties that are at least equivalent to those of molded articles prepared by prior methods.